This invention relates to an apparatus for controlling the inside air temperature of a motor vehicle. The apparatus includes a cooling unit, a heating unit, dehumidifier means for drying the air, and control means which includes circuitry for compensating for temperature changes induced by operation of the dehumidifier means.
An air conditioning arrangement of this type is disclosed in, for example, German published application (DE-OS) No. 29 52 210. This publication describes the basic arrangement of an air conditioning system consisting of a cooling unit having a compressor and an evaporator, a heating unit, and means provided for controlling the heating unit and the compressor of the cooling unit. To dry the air inside the motor vehicle, dehumidifier means can be provided which, when activated, effects the turning on of the compressor independently of the control settings of the air conditioning unit. The moisture in the damp air flowing through the unit is condensed out at the evaporator lamella.
Unless the cooling unit is already operating, the switching on of the compressor by the activation of the dehumidifier means effects a reduction in the inside air temperature due to the through flow of the cooler air. Because the normally used proportional controller for controlling the air conditioning system requires a finite control differential prior to delivery of an output signal, the effective temperature reduction caused by this operation of the compressor is not fully compensated for. In some cases, depending upon the size of proportional regions, the required control differential is approximately two degrees to four degrees centigrade (2.degree. C.-4.degree. C.). Even modern controllers can have large proportional regions requiring differentials in the range of up to 8.degree. C. Accordingly, (DE-OS) No. 29 52 210 provides switching means for adjusting the desired temperature control value simultaneously with the activation of the dehumidifier means. This adjustment of the desired temperature value effects an increase in the vehicle inside air temperature to a middle value that is selected so as to approximately compensate for the effective temperature reduction caused by the operation of the compressor.
However, a disadvantage of this arrangement is that this increase in temperature to a middle value is exact only under certain operational conditions of the air conditioning unit. Under other operational conditions, the resulting temperature increase, which is always a constant value, is too large or too small since the effective temperature reduction resulting from the operation of the compressor is not completely or is too strongly compensated for. A further disadvantage of this known arrangement is that the adjustment in temperature takes place even if the cooling unit is already operating in the "full cooling" mode. Under this condition, the activation of the dehumidifier means does not effect the operation of the cooling unit because the compressor is already running. Therefore, there is no resulting temperature reduction inside the motor vehicle. Nevertheless, the temperature control adjustment is made which leads to a corresponding increase in inside air temperature. The same problem exists if the outside temperature goes below 0.degree. C. To prevent icing of the evaporating lamella, control circuitry discussed below is usually provided to prevent the compressor from running when the temperature is below freezing. A command to switch on the compressor effects, in this case, no further air temperature reduction, yet the constant value temperature control adjustment is made, leading to an increase in the inside air temperature of the vehicle. An object of this invention is to provide an air conditioning unit of the above-mentioned type which avoids these disadvantages. Specifically, the amount of temperature compensation provided should not be a fixed middle value, but rather should depend upon existing driving or operating conditions, i.e., the outside air temperature.
These problems are inventively solved through a circuit corresponding to that shown in FIGS. 2 and 3. As shown there and described below, this invention achieves compensation for the temperature reduction caused by the operation of the compressor upon the activation of the dehumidifier means in an amount which is dependent upon the outside air temperature and which corresponds to the exact temperature reduction which would otherwise occur. It is also achieved, independently of the operating condition of the air conditioning unit, that there is an exact compensation for the temperature increase caused by the activation of the dehumidifying means device.
Upon activation of the dehumidifier means, it is desirable to additively combine the value measured by the outside air temperature detector with a control variable to effect compensation of the switching means controlling the temperature inside the motor vehicle. This additive combination of a control variable with the value measured by the outside air temperature detector can advantageously be performed by a resistance network. This method effectuates a simple solution while utilizing low cost components.
An additional advantageous feature of the invention is that the control variable is allowed to affect the heater controls only if the cooling unit is not already running at "full cooling" when the dehumidifier means is actuated. This is indicated by the absence of a signal at the point in the switching circuit which controls the compressor. With this feature it is possible to prevent an undesired temperature increase. When the compressor of the cooling unit is already running, connection of the dehumidifier means effects no temperature reduction and, thus, compensation would lead to an undesired increase in the inside air temperature of the motor vehicle.
These functions can be effected with a logic circuit. An advantageous arrangement for this purpose includes an operational amplifier, two resistance networks and two diodes. The operational amplifier can be connected as a comparator whereby the value measured by the outside temperature detector is combined with the control variable through a diode only if the dehumidifier means is activated and the cooling unit is not running at "full cooling".
Furthermore, it is especially advantageous to provide additional switching circuitry which allows for connection of the control variable to the heating controls only if the outside temperature is above 0.degree. C. An undesired temperature increase would otherwise result because the command to switch on the compressor, in this case, leads to no further reduction in the temperature of the through-flowing air. It is desirable to construct this additional switching circuit as a resistance network with a diode. This solution exhibits simplicity and economy.
Further objects, features, and advantages of the present invention will become more apparent from the following description when taken with the accompanying drawings which show, for purposes of illustration only, an embodiment in accordance with the present invention.